Very large amounts of synthetic resins are employed as protective coatings on metals. A high proportion of the resins consumed for this purpose are applied to metal surfaces as solutions in non-aqueous solvents which are subsequently evaporated. Pollution of the atmosphere with the resulting vapors is highly undesirable and their recovery or combustion is a very expensive alternative. Consequently, methods of resin application which do not require the evaporation of large amounts of such solvents are being sought. A particularly attractive such method is to apply the resin as a solution or dispersion in water, a solvent which does not have to be recovered or burned after it is evaporated off.
It has been found particularly difficult to provide aqueous coating resin systems which can meet the requirements for suitable linings in food and beverage cans. Such linings function to chemically insulate the can and its contents from each other. The lining resin frequently is applied to the can metal as a thin layer and cured before the metal is fabricated into a can. Thus, to be suitable as a can lining material utilized in this manner, the cured resin not only must be inert and impervious to the contents of the can but also must be able to adhere to the metal and to retain its integrity under the conditions involved in can fabrication and in cooking, pasturization, etc., of the contents. Additionally, the lining material must not have any substantial toxicity and must not impart any objectionable taste or odor to the can contents.
Several types of resins which meet all of the foregoing requirements and are soluble (before curing) in non-aqueous solvents have been developed. The difficulty in utilizing such resins as aqueous systems is that the structural parameters which are essential to the required properties of the cured resins apparently would have to be altered in order to attain water dispersability. That is, the cured resin must be essentially hydrophobic, or at least not water-sensitive, but the uncured resin must comprise hydrophilic functional groups.
A particular group of resins which have proven to be excellent for can lining (and other coating) applications are nominally-difunctional, epoxide terminated, linear polymeric adducts of bis-phenols with their diglycidyl ethers. Curing agents conventionally employed with these resins are exemplified by condensation products of formaldehyde with ureas, phenols or triazines. The resin and curing agent are dissolved in an organic solvent, such as xylene, lower ketones, higher alcohols or glycol ethers, together with catalysts, accelerators, etc., and then applied to the metal substrate and baked.
The most widely used resins of the foregoing type are DGEBA (diglycidyl ether/bis-phenol-A) resins, i.e., polyether diepoxides derivable from the polymeric adduction of bisphenol-A ##STR1## with the diglycidyl ether of of bisphenol-A ##STR2## The diglycidyl ether may be performed by reacting two molecules of epichlorohydrin with one molecule of the bisphenol-A in the presence of a base, such as sodium hydroxide. Classically, however, the latter reaction is carried out in such a manner that the resulting di-ether molecules react in-situ with bisphenol molecules to produce the DGEBA resin.
In the latter case, the reaction product tends to be a mixture consisting predominantly of polymeric species of different molecular weights corresponding to different values of n in the following idealized formula: ##STR3## By reason of including some monofunctional epoxide species, such mixtures exhibit average epoxide functionalities of somewhat less than two.
In order to impart satisfactory properties, for can lining applications, to cured resins of the DGEBA type it is generally considered that the average molecular weight of the uncured resin must be within the range of from about 2,000 to 10,000 (n=6-34). Optimum properties are generally obtained at an average molecular weight of about 4000 (n=.about.12-14). Despite the formation of a secondary alcoholic hydroxyl group for each epoxide (oxirane) ring reacted in the polymerization, the extent to which these resins can be dispersed in water drops off very rapidly as their molecular weights go up. Thus, even at n values as low as 0 (mol. wt. about 340), the preceding resins are not directly water-thinnable and must be emulsified in order to be dispersed in water. Such emulsions are not easily prepared and often do not readily yield coatings as good as those obtained from non-aqueous solutions of the same resins.
According to U.S. Pat. No. 2,541,027, resins of the preceding type, when mixed with phosphoric acid, applied to a metal substrate and heated (or allowed to stand several days), cure to coatings which adhere strongly to the metal, have high chemical resistance and are flexible, hard and tough. The cured resins are said to be particularly useful as primer coatings on ferruginous metals.
The '027 patent does not disclose water-dispersible products as being formed from the reactions of epoxides with phosphoric acid and, in fact, it has not been found possible to render DGEBA resins having average n values substantially higher than about 0 directly water-thinnable, simply by reacting them with phosphoric acid.
Water-thinnable, thermosetting resin systems are known in the coatings industry. For example, "alkyd" type polyester resins are made water soluble by neutralizing residual carboxylic acid groups attached to the polymer backbone with suitable amines. The same method has been used to solubilize other types of polyester resins and certain esters produced by the condensation of epoxy resins with dimeric fatty acids. However, the foregoing water-thinnable systems have not found commercial employment in such coatings applications as can lining.
U.S. Pat. No. 2,723,971 discloses coating compositions made by reacting phosphoric acid with the epoxide groups of copolymers of ethylenically unsaturated epoxy monomers and ethylenically unsaturated, acyclic epoxy-free monomers, the polymerization being of the free radical initiated, addition type. Depending on the nature and proportions of the component monomers in the copolymers and on the relative amounts of ##STR4## in the reaction products with phosphoric acid, the resultant coating composition may be soluble in aqueous alkaline solutions. That is, some of the disclosed compositions are water soluble as such or may be rendered water soluble by neutralizing the diacid phosphate moieties they contain with amines, ammonia or other inorganic bases. An oxirane oxygen content in the copolymer of at least 3.9 wt. % (an average epoxide equivalent weight of 410 or less) is said to be required to render the phosphated product alkali-soluble, although fewer neutralized acid phosphate groups are said to be required when other water-solubilizing groups, such as acyloxy or beta-methoxyethyl acrylate moieties are present. Typical unsaturated epoxy monomers employed in the compositions of the '971 patent are allyl glycidyl ether and glycidyl methacrylate.
The coating compositions disclosed in the '971 patent are cured by air drying or heating, presumably as a result of oxidative cross-linking between double bonds.
The alkali-insoluble compositions of the '971 patent are taught as being preferred, by reason of having the best combination of properties, particularly for coating applications. Further, the '971 patent teaches, in effect, that copolymers of the type disclosed provide for the combination of a high molecular weight with a sufficient oxirane content to ensure that the neutralized reaction point with phosphoric acid will be water soluble, but that this combination cannot be obtained with epoxide-containing polymeric ethers.
U.S. Pat. No. 3,975,346 teaches the DGEBA resins can be converted to water-dispersable coating resins by reaction with quaternary ammonium salts of acids, including phosphoric acid. However, the presence of boron (as boric acid, for example) is said to be essential to adequate cured coating (film) properties. Furthermore, the epoxide groups in the DGEBA resin are reacted with the amine components of the quaternary salt (as such or prior to salt formation) and no epoxide/acid reactions appear to be involved. Also, the cured coatings are not indicated to be water resistant. An additional consideration is that the disclosed coatings, containing up to 8% boron (which is toxic), would not be acceptable as can linings. The only relationship between EEW and water-dispersibility disclosed in the '346 patent is that which may be inferred from a teaching that the resin preferably contains at least about 0.05 percent by weight of nitrogen in the form of chemically bound quaternary ammonium base salt groups. (On the basis of one nitrogen per oxirane group originally present, this is equivalent to a maximum EEW of about 280-14, or .about.266.)
It thus does not appear that a way to effectively render DGEBA type resins water-thinnable without introducing water sensitivity and/or toxicity in the cured resin has been provided by the prior art.